FIFTH CANADIAN CONFERENCE ON NONDESTRUCTIVE ... - IAEA

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INTRODUCTION
X-ray diffraction is one of a few nondestructive techniques for
measuring stress in engineering components and structures. The basic
principle is simple and techniques using X-ray sensitive film for measuring
stress in crystalline materials were developed more than 40 years
ago (1). Recent developments in electronics and X-ray detector technology
have allowed a reduction in the size of the equipment required and
an increase in the speed with which accurate measurements can be made.
Furthermore, an improved understanding of the relationship between the
stress state in a polycrystal and the distortions measured by X-ray diffraction
allows a more reliable interpretation of data than was previously
possible (2,3).
THE PHYSICS OF X-RAY DIFFRACTION
The measurements of stress by X-rays is based on the Bragg law
for the diffraction of monochromatic X-rays by the planes of a crystal,
i.e.:
nX = 2d sin 6 Eq. 1
where: n is the order of diffraction
X is the wavelength
d is the interplanar spacing of the diffracting planes
e is the diffraction angle
Unique values of X are obtained by making the anode of the
X-ray tube from a pure metal ; the X-ray wavelengths normally used correspond
to the transition of electrons from the L to the K orbitals of the
atoms in the anode. The nature of diffraction from a three dimensional
crystal lattice is such that the X-rays appear to reflect at a unique
angle (S) corresponding to a unique interplanar spacing (d). As ö
approaches 90° - the back reflection range - changes in d corresponding
to elastic strains of less than 10"^ can be measured with acceptable
accuracy.
The X-ray wavelengths useful for diffraction in metals are in
the range 0.15-0.25 nm and these have a penetration depth in most metals
of a few hundredths of a mm. Hence X-ray diffraction is suitable for
measuring elastic distortions at the surface.
PRINCIPLES OF MEASUREMENT
We define a set of axes x, y and z in which z is normal to the
specimen and x and y are aligned with some axes of symmetry in the specimen
(Fig. 1). In practical terms we can measure the lattice spacing in
any direction, m, within a cone 5O~60° from the specimen normal. We